Need Help Tuning Navien CH-180 ASME Mod Con Combi

I'm new to the forum. After lurking around a little and finding a tremendous amount of good information, I decided to join.

Recently, a Navien CH-180 ASME modulating condensing combination boiler was installed to replace my original oil-fired boiler. Iâ€™m looking for feedback and advice on setting the boiler up for efficient operation. The installer is not educated regarding this â€“ they simply set the supply temp to 160 F and walked away, which is unfortunate, so Iâ€™m on my own. After some quick research, I immediately set the supply down to 140 F, but now I have become more educated and want to take full advantage of the new boiler.

Right now I have the optional Outdoor Temperature Sensor installed and I have the Supply Temperature K-Factor set to 1.4. (Navien calls their outdoor reset a K-Factor for some reason.) To simplify things, I am not using Zone 2 right now. My burn times range from about 12-18 minutes, with about 9 minutes in between. I have nice even heat without any temperature swings (as a matter of fact, the reading on my thermostat never varies from the t-stat set point during steady-state operation). During the burns, the boiler modulates down, always stays firing (no short cycling), and the supply temperature seems to track slightly below the K-Factor curve as specified by Navien.

Whatâ€™s interesting to me is that the boiler does not seem to be producing any appreciable amount of condensate over the last few days. I put the drain hose into a 5-gallon bucket when I started running on the outdoor reset a few days ago, but it has only produced about Â¼ cup of condensate in that time. Daytime temps have been in the very low 30s with nighttime temps in the mid teens. Surely Iâ€™m loosing condensate to evaporation out of the bucket, but I guess I was expecting to see more condensate.

Here are my questions and topics for discussion:1) Should I try to lower the K-Factor even more? 2) Is there any benefit to trying to achieve even longer burn times by lowering supply temperature even more?3) Are there adjustments that should be made to my Honeywell programmable thermostat? Right now, the cycle rate is set to 3. Iâ€™m not sure exactly what this does. However, would lowering this to 1 give me more â€˜lagâ€™ â€“ resulting in longer burn times as well as longer off times? Would this be beneficial?4) Why would I not be getting more condensate? Is it just that I am at the high side of return temps, and I need even lower return temp to condense more.

Now, I also need to start thinking about what Iâ€™m going to do with Zone 2, which is my downstairs room on a concrete slab. It is 1 bathroom and a kidâ€™s play room. Baseboard is installed in the bathroom. The Myson Whispa III 9000 was installed in the main room due to ease of installation. It has a fan that comes on when supply temp hits 110 F. So, when the supply was set at 140 F, the fan would always come on when Zone 2 called for heat, and the room would heat up nicely. Now that Iâ€™m operating on outdoor reset, if Zone 2 calls for heat when its 41 F outside, chances are the supply temp wonâ€™t be hot enough to kick the fan on and Iâ€™ll have the equivalent of about 1 foot of baseboard (if Iâ€™m lucky). Any suggestions? Due to the layout of the room & being on a slab, Iâ€™ve been told by several people that installing baseboards would be very labor-intensive ( = $$$).

In order for the boiler to produce much condensate (and get full condensing efficiency out of it) the water returning water from the baseboard has to be entering the boiler at temps significantly below 125F (the very beginning of condensing.) That generally takes an output temp under 140F or lower.

Set the thermostat up to run at a constant temp, and keep lowering the K-factor until it either doesn't quite keep up, or it starts short-cycling. Then bump it back up one notch.

The min-fire output of the -180 ASME is about 16,000BTU/hr, so it will balance perfectly with your 80' zone at 200 BTU/ft, which happens when the output temp of the Navien is about 125F, (returning water about 115F) which is good- it won't short-cycle at temps well into the condensing range.

But the other zone could be an issue. According to the manual , with an entering temp of 140F the Myson only delivers ~6000BTU/hr, and that's at the higher blower speed. You may need to upgrade the radiation on that zone to take full condensing advantage, but see how it goes. When you bump the K-factor down so that you're guaranteed to have output around 130F, turn the Zone-1 T-stat way down (or off) on the other zone, and time the burns with just zone-2 calling for heat. If it's under 1 minute with 130F output on the boiler it's a problem but if it's 3+ minutes, you're fine.

All good solutions start with a heat load analysis, and we're kinda shooting in the dark here. A typical 1990s code-min 2-story comes in with a heat load at 0F at about 15BTU/foot. The 99% outside design temp for Windor Locks is +8F, so all things being equal ~13BTU/foot might be a middle-of-the road WAG, which would imply a load of about 27,000BTU/hr. Depending on how much of the raised ranch is below grade, and the depth of the cantilevers it could vary by quite a bit from that crummy rule-of-thumb approach, it's probably not more than 35K worst-case, and it could be close to 20K.

If it turns out to be close to the 27KBTU/hr number you should be able to run in condensing mode literally all the time and still keep up (without short-cycling) on the fin-tube zone, so that K-factor can probably be dropped by quite a bit- be pretty aggressive in your initial adjustments. (At 130F AWT you'll get over 20,000 BTU/hr out of 80' of fin-tube, which is MOST of your heat load.)

Luckily, I did do some testing on just the downstairs zone (Zone 2) when the supply temp was set to 130 F with an outside temp of 46 F. With Zone 2 calling for heat the whole time, the boiler short cycled; I timed 4-5 minute burns with about 12-13 in between. The supply temp would drop to about 105 F, the burner would fire, supply would rise to 131-133 F, the burned would shut down, and the whole process would repeat. I just assumed that any short cycling was bad.

So, that should tell me that Zone 2 should operate OK if my supply temp is around 130 F or above. I'm worried if I get aggressive with the K-Factor adjustment, then the supply temperature won't be 130 F until the outside temperature is around, say, 10 F. That means the Myson won't be doing much most of the time. But, like you said, I guess I'll have to see how it goes.

I'm curious to see how low I can push the K-Factor until it short-cycles or can't keep up. For years, I cursed my abundance of baseboards on Zone 1 and kept the louvers shut down to avoid temperature swings and overheating some rooms. Who would have thought that they would be my savior. It's interesting to me to see how low I can run for supply temps and still keep the house comfortable.

When trying to use the lowest K-Factor, is the best thermostat strategy to set a comfortable temperature on the t-stat and just leave it there? We like it a little cooler at night when sleeping, by a few degrees. I have been setting the t-stat for 70 F during the day and 68 F at night. To me, it still feels a little cool in the morning even though the air temp is 70. (With the old boiler, t-stat was set at 70 F daytime and 64 F night, with the set-back primarily intended for $ savings.)

A 4-5 minute burn on a low-mass boiler serving up only 4 burns/hr is not a short cycle. When it's 10+ burns/hr and under 3 minutes/burn, that's a problem. Odds are that even if it short cycles a bit with just zone 2 calling for heat, much of the time calls for heat from zone 1 will overlap, especially if the output temp is low enough that it takes a long time to satisfy the zone-1 T-stat.

Setting the thermostat to a constant temp and lowering the boiler's output temp to where it just keeps up is where the best combustion efficiency is achieved. When you have the ODR curve set that low, recovery from even a 5F setback could potentially take hours. But if you bump the curve up to be able to use setbacks, you often give up more in combustion efficiency than you save in lower temp. eg: If setback saves say 5%, but the higher temps needed to achieve a reasonably recovery ramp cuts your average efficiency from 95% down to 89%, you've lost ground. There are a few boilers out there that automatically over-ride the ODR to speed up recovery ramps from setback, but I don't think the Navien combi systems have that function. (Read the manual, just to be sure.)

Whether a constant-temp/low-output approach burns less fuel for you than bumping the curve a bit and using setback strategies depends a bit on your occupancy patterns. A 2 person household where both of you are working outside the house and are gone from 8AM-8PM every day might do better with a setback strategy. A 2-adults 2 kids under 5 with a stay-at-home parent would surely do better with at constant temp/lower curve strategy.

If you're a tech geek with somewhat variable schedules, and like to control your world with an app on your smart phone, you could do worse than buying a Nest thermostat, which "learns" your occupancy patterns by when you bump it up/down over several days, and has the ability to be adjusted remotely if you decide you're coming home early and want the house to be warm when you get there. It also adjusts the start of recovery ramps based on how long it took on recent recovery setbacks, so you can set the ODR curve fairly low and still live comfortably with a setback strategy. Street price is about $250- not a cheap toy by any means, but it works for some people.

If you have a mid or late-winter oil bill with a "K-factor" (with different definition than how it's used in the Navien documentation) stamped on it, that would be enough information to estimate fairly closely what the heat load is, using the oil boiler as the measuring instrument.

Doing a somewhat formal heat load calculation could still be useful, if you decided you really need to bump the zone-2 radiation to squeeze more efficiency out of the Navien. Even a room by room I=B=R method of both zones would be fine, sufficiently accurate to match the radiation/load ratios reasonably when you make the adjustments. I suspect the reason you have enough fin-tube on zone-1 to heat the place with 130F AWT even under the 99% outside design temp had more to do with not short-cycling the (guaranteed to be at least 2x) oversized oil boiler than with the actual heat load. At 180F AWT (typical for heating with oil boilers) that 80' of fin-tube is dumping 50KBTU/hr into the zone, which it wouldn't actually NEED unless it were 50 below outside. But it's enough to keep an oil-fired ~60K output 3-plate cast iron beastie from short cycling itself to death.

Thanks for all the info Dana. It's so nice to get some useful info & feedback. My installer just looks at me like I have lobsters coming out of my ears when I ask an efficiency/tuning question.

I think we are defining a short cycles a little differently - I think of a short cycle as any time the boiler cycles while the t-stat is still calling for heat, regardless of how long that cycle is. I'm looking at it differently now.

The Navien doesn't have a ODR over-ride. My wife stays at home, and I have 2 grade-school children, so I'm going to try to adopt the constant temperature strategy as best I can.

Thanks again for your help. I'll start reducing the K-Factor and see what happens.

So, it looks like my burn times were steadily creeping up. Not wanting to stay up all night to see what happened, I set the K back to 1.4. Now, I went back this morning and looked at my K=1.4 test numbers. My run times were creeping up on that as well. Over a 1.5 hour period from 10:45pm to 12:15am (T-stat set to 68F with 25F outside): 15 min burn, 9 min off, 15, 9, 17, 9, 18.

Then I got thinking. I have a significant length of baseboard located in the 2 kid's bedrooms. When they go to bed at around 7:30-8:00pm, their doors are almost closed. I think this is 'taking away' baseboard from the rest of Zone 1, and that's why my burn times are increasing. Their doors were closed during both of my tests. I typically open their doors when I go to bed. I did notice their rooms seemed slightly warmer when I opened their doors before I went to bed last night.

What do you think? Have I found my limit? Should I set it back to 1.3 this weekend when I have more tome to keep an eye on it?

My other concern is DHW. The CH-180 gives priority to DHW. It is not uncommon to have a few showers back-to-back along with a few loads of laundry early in the morning. So, it is possible to have a significant amount of time at the coldest part of the day when the boiler isn't supplying hot water to the baseboards. I'm worried that my heat will 'fall behind' and never be able to catch up.

You haven't actually found the limit but it might be getting close. When the heating system can't maintain temp (or it starts short-cycling during milder weather), THAT is the limit. When it's actually cold out the output temps of the boiler are higher, and the BTU/hr output of the radiation is higher, so you will see a higher duty cycle like that, but that doesn't mean it won't still keep up with water 5-10F lower than that. (With 110F return water temps you're definitely into the 90s efficiency wise, but not the high-90s (which you can probably hit with your zone-1 radiation if you dial it all the way in.)

When the the ODR curve is EXACTLY dead-nuts on, the thermostat is essentially never satisfied during the night (when there is no solar gain and most of the lights are off), the system runs constantly, but the indoor temps stay within a degree or two of your setpoint. Then when the sun rises &/or people get up and start turning on things that use power, the temps rise enough to trip the T-stat.

You needn't worry about the heating system falling behind during back-to-back showers. Let's say it's truly cold morning, with no sun and the heat load on the house is running 24,000 BTU/hr. Then the shower kicks in for straight half-hour- that's a net 12,000BTU/hr that didn't go into the baseboards, but some fraction of that 70,000BTU/hr shower (around 10%) ended up as heat in the house near the bathroom. The net loss to the house was then maybe 5000-6000 BTU/hr. Running a hair drier for 5 minutes puts another 300BTU or so back into the house, put on a pot of coffee and you're adding another 500, etc.

The thermal mass inside the house is pretty substantial- to lose even ONE DEGREE to a ~5000 BTU shortfall on the heat load you'd have to be living in the least- massive house in CT! Doing a small amount of napkin-math: A raised ranch lower floor is usually fully conditioned heating space that has concrete slab under the finish floor. A 750' 4" slab has 250 cubic feet of concrete, which in total weighs about 35,000lbs. The specific-heat of concrete is about 0.2 BTU/degree-lb, so for that concrete to change one degree takes (35,000 x 0.2=) 7000 BTU. You also have the wallboard, wood, etc, which also have significant weight & specific-heat that contributes substantially to the thermal mass inside of conditioned space, which is why it takes some time to either heat up or cool down even when the heating/cooling equipment's output is 2-3x the instantaneous load.

This weekend is the perfect time to experiment to find out how low you can go on the ODR curve and still keep up, since the outdoor temps overnight will be dropping pretty close to the 99% outside design temp. When you have found the bottom of where it can't quite keep up with your thermostat setpoints, bump it up one notch. You then have to pay attention to the system behavior when the outdoor temps rise into the 40s or higher, since that is when the output temp of the boiler and output BTU/hr of the radiation will be much lower, making it more susceptible to short-cycling.

My suspicion is that zone-2 will have the hardest time keeping up, since it has a substantially lower radiation/conditioned floor area ratio, but since heat loss is a function of exterior surface area (and not floor area), I could be wrong. (That's why room-by-room heat load calculations are really called for.)

This weekend is the perfect time to experiment to find out how low you can go on the ODR curve and still keep up, since the outdoor temps overnight will be dropping pretty close to the 99% outside design temp.

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OK, I'll do that. I think I'll set it back to 1.3 and see what it does over a few days instead of just a few hours.

Looking ahead to temps in the 40s: If I get short cycling, is the only real solution to add more baseboard?

Zone 1 looks like I have room to add another 12-16 feet of baseboard by simply connecting 2 existing baseboards with another section of baseboard. I wonder if it would be worth it to do this.

Zone 2 looks like it will require additional baseboard no matter what (if I intend to operate this boiler as efficiently as possible). Too bad, since the installer put in Zone 2 at the same time the new boiler was installed. The Myson Whispa was presented as much more economical than adding baseboard. I wish I had opted for the baseboard.

You won't have to add more baseboard to zone 1, but if the Myson only kicks on at 110F you may have to do something about zone 2 when the outdoor temps are milder.

Baseboard probably isn't the right choice unless there is room to add a lot of it, and you would have to do some sort of room-by-room load calculation on that zone to make sure to but the right amount in each room to get it the temperatures reasonably balanced. If you are willing to pay a bit for comfort there are some relatively low cost panel-radiator options out there, that have the additional benefit (from a short-cycling suppression point of view) of having at least some water volume in them for thermal mass. You only get about a gallon of water out of every 40' of 3/4" copper core baseboard, half that for the half-inch stuff, which isn't a lot of thermal mass to work with. Panel radiators of equivalent output will have about 5x the thermal mass of 3/4" plumbing baseboard, 10x the thermal mass of 1/2' plumbing baseboard. And since a reasonable fraction of the output is radiated rather than convected, it's more comfortable. But even the lower cost rads are on the order of 2-3x the cost of 2" fin-tube convecting baseboard solutions, on a BTU-output basis.

But let's not bother with the details until we know it's really necessary. Most of the time I would expect the calls for heat to overlap, so even if it short-cycles when serving ONLY zone 2, if most of the time it's simultaneously serving zone 1, the most burns will be of reasonable length making the burns per hour or burns per day manageable, even if there is the occasional short-cycle.

Rather than setting the K-factor to 1.3 again, be a bit more aggressive- before you go to bed set it to 1.2- you won't freeze, and if you wake up to a fully warm house that is close to the setpoint, you know you can bop it back another notch. But if you wake up and the house is 5F colder than the setpoint, you'll know that 1.3 is really the right K-factor to use and you're done. Looking at the output curves in the manual, a K-factor of 1.2 would deliver 140F out of the boiler at about a +5F outdoor temp. With 140F out, 120F back (an average temp of 130F), 80' of baseboard delivers over 20,000 BTU/hr into the zone, which may be above the actual +5F heat load for that zone. With the K-factor set to 1.3 it delivers 140F output at an outdoor temp of 14F, which is above your 99% outside design temp, and by the time it hits your 99% temp you'll be drifting out of the condensing range.

With a calculated heat load number we could better predict where the right K-setting is, but it's probably going to be fine at 1.1-1.2 for zone 1, then you'll have to figure out what to do about the other zone, since the Myson might not be kicking on the blower unless it's below freezing outside with the K-factor that low, since the water temp may not be enough to reliably trip it's aquastat. If that's the case, you could just set up the boiler for a fixed-temp output of 130F out until you deal with replacing the Myson with non-powered radiation.

If you want to try running a I=B=R type heat-load calculation tool, the bare-bones Slantfin tool is good enough to proportion your room-by-room radiation. (It's actual heat load numbers are usually higher than measured reality- not insanely high, but not close enough for dialing in a mod-con to it's max efficiency.) If you let the tool pick the baseboard lengths they will be on the stubby side for a mod-con, but you can look at it's baseboard recommendations relative to what you actually have for zone-1 baseboards, and use a similar multiplier factor to come up with the radiation for zone-2. That way both zones will work about the same at the same K-factor. The manual for the tool lives here.

... but if the Myson only kicks on at 110F you may have to do something about zone 2 when the outdoor temps are milder.

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I contacted a guy at Myson who suggested bypassing the temp switch and powering the Myson any time Zone 2 calls for heat. I'm not sure how to do that, or if it can even be done. I first thought that I could use the Zone 2 power to the circulator, but it's a single circulator, jumpered to operate when either Zone 1 or Zone 2 calls for heat. Still thinking about this, but if you have any suggestions, I'm all ears.

If you are willing to pay a bit for comfort there are some relatively low cost panel-radiator options out there that have the additional benefit (from a short-cycling suppression point of view) of having at least some water volume in them for thermal mass.

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From reading some of your (and others') posts, I had already started looking into the panel radiators. I'm just not sure how to go about figuring out who to go to for installation of these. I guess I could DIY, but I'm a mechanical guy - I enjoy wrenching on cars, but I have no desire to start tearing apart my downstairs to install panel radiators. I'd rather leave it up to a professional. Turns out my problem always seems to be finding a real professional.

When I get some time, I'll have to try the I=B=R tool. Thanks for the links.

Rather than setting the K-factor to 1.3 again, be a bit more aggressive- before you go to bed set it to 1.2- you won't freeze, and if you wake up to a fully warm house that is close to the setpoint, you know you can bop it back another notch.

After setting the curve to 1.2, I ignored the boiler, which is tough for me because I'm really curious to see how my changes affect how the boiler runs.

Anyhow, I woke up this morning and checked things out. I have an indoor thermometer in my living room that agrees very closely with my thermostat. It showed that the indoor low last night was 68F, and it was about 68.5 at 7:30am. Now, I have my t-stat set to bump the heat up to 70F at 5:30am. Right now it's 8:00am, t-stat set at 70F, and my indoor temperature shows 70.3-70.5F on my thermometer and 70F on my t-stat.

The boiler appears to be cycling with calls from the t-stat only. I checked the supply temp a few times, and it's about 135-138F. That seems to track with the K=1.2 curve for 14.5F - maybe a little on the high side.

Sounds like you're in pretty good shape on the curve-tweaking then. During warmer outdoor temps/cooler-boiler output the radiation doesn't have enough output to keep it from cycling, but it looks like the boiler's hysteresis set to limit the water temp overshoot from the curve setting to about ~15F, and waits until it's undershooting by ~15F for about a 30F total differential.

The burn times will be shorter when it's 50F outdoors, but you'll probably still be look at only 4-5 burns/hour, not 10. You may be able to observe that later this week, if the regional forecasts for mild weather on Thursday/Friday pan out.

I'll bet you're getting a decent volume of condensate out of it at the lower operating temps too.

Fan coil units like the Myson are usually set up with internal aquastat controls to avoid the wind-chill issues. You can heat a room to 70F with 80F air easily, given enough air volume, but it's not very comfortable for humans directly in the air-flow. You may be able to retrofit the blower control with an aquastat that comes on at 100F, which is about the lower limit of wind-chill comfort, being that 100F is about human body temp. A flow of 90F air feels pretty tepid, and below that feels downright cold. If the zone controller/relay controlling the circulator is rated for enough additional current (probably is) you COULD just run the blower in parallel with the circulator, but when your water temps hit 98F you'll be well down the curve for being comfortable sitting in front of the thing, and when only zone 1 is calling for heat with no water flowing in the zone 2 plumbing it'll be even worse. Bottom line, it's not a very good hack. Retrofitting an adjustable aquastat that can go as low as 90F or lower is probably the better bet.

The burn times will be shorter when it's 50F outdoors, but you'll probably still be look at only 4-5 burns/hour, not 10. You may be able to observe that later this week, if the regional forecasts for mild weather on Thursday/Friday pan out.

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The warmer weather operation is definitely my concern now. I guess I'll just wait for warmer weather to see what my burn times look like. I'l report back on this.

If the zone controller/relay controlling the circulator is rated for enough additional current (probably is) you COULD just run the blower in parallel with the circulator...

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That was my original plan since the Myson draws very little current - like 1/2 to 3/4 amp. However, I only have 1 circulator, and the Zone 1 and Zone 2 outputs to the circulator are jumpered together at the controller. I think this means that running the blower when only Zone 2 is on can't be done, correct?

You could run the blower when zone 2 calls for heat by adding a zone-relay with it's control input in parallel with the zone valve, but I'm not sure it's worth it. Running a room-by-room heat load calc and replacing the Myson with a panel radiator with output that balances with the rest of the zone or house would be a better use of funds, unless you just had the zone relay kicking around in your parts bin or something.

The weather has been warm here, and I definitely found the limitations of the Navien K-factor curves. I woke up this morning to a 66F house, even though the t-stat was set to 68F. It was about 52F outside. The t-stat was calling for heat for several hours, but the boiler never fired, even though the supply temp was down to 68F. I had to bump the K factor up to 2.0 before it would fire.

Unfortunately, the Navien makes me pick from pre-set K-factors, and I can't set my own curve.

I'm thinking about maybe changing the setting to a return water setting instead of supply, and just leaving it for the time being. I was going to try 120F return water setting and see what it does. Worth a shot I guess.

Anyhow, I'll probably be taking a break from tuning this rotten Navien. It's been 3 months since it was installed, and it took about 6-8 weeks for the installer to get it to the point where it seemed to run good. However, I'm still battling a weird vibration/buzzing coming from the main gas valve, which vibrates the whole back of my house. I just got notice from Navien that they will be replacing the entire boiler, so I'll have to start over once the new unit is installed.

The most likely reason it doesn't keep up during warmer weather is that fin-tube baseboard output is extremely non-linear with water temp at temps below 110F. This is because the convector height is only ~6" with baseboard, which isn't enough stack-effect to move much air volume when the fin-tube is at bath-water temps.

Setting it up the curve to work off the return water temp is probably going to fix that issue. But rather than 120F, set it at 100-105F. That is plenty high enough so that the aquastat on the Myson will trip, and under sustained calls for heat you'll still be putting out sufficiently high temps to cover the load while still condensing, assuming it still allows the +15F overshoot while cycling the way it did on your prior curve tuning experiments. If you set it up at 120F it'll only be condensing half the time as it cycles, since anything more than a 5F overshoot would take it over the condensing zone.

No, no explanation of why they are replacing the entire unit. In general, Navien has been fairly responsive to my questions, but they don't explain much when it comes to troubleshooting my boiler. I'm an engineer, so I understand how the unit works and I want to know what's going on. I believe that the gas valve buzzing is due solely to gas supply (since we are on the 2nd gas valve and they both make the same noise), so I have no idea why an entire new boiler will solve my issue.

I tried setting the unit to 120F return temperature with high hopes. The problem is that on a call for heat, the burner fired as hard as it could to try to get 120F return as quickly as possible. I have the max heat input limit set to 50% of maximum, but the supply temp still shot up to about 175F fairly quick, then the return temp would overshoot and the burner would shut down. I might try setting it at 100F and see what happens.

EDIT: Just tried setting to return temp of 105F:
Burner fired for less than 2 minutes and shut off due to overshooting the return temperature at 120F.
Burner off for 4 minutes
Burner fired for less than 1 minute when it shut off at 120F return
repeated 4 min off, 1 min on cycle

Now, Navien just had my contractor replace the PCB board, main power transformer & wiring harness - all in an attempt to solve the gas valve buzzing problem. They never checked & re-set the gas/air pressure settings per the manual - apparently the Navien Tech Line (telephone) told them it wasn't required. Now I'm having intermittent ignition issues and the burner seems to 'hunt' after it modulates down to maintain supply temp. I'm hoping they schedule the boiler replacement soon, because I'm not too confident in the way it's operating now.

I'm very frustrated at this point. I'm at the point where I'm ready to tell them to take the Navien out and replace it with something else. Only problem is that the Navien CH180ASME seems to be well-suited to my heating requirements, since it modulates down to about 16,000 BTU/hr. It looks like other modulating/condensing/combination boilers don't have turndowns that low. If I were to replace this with something else, do you have any suggestions?

It's OK to let it go to high-fire for bringing the return water temp up quickly as long as the burn times are reasonably long. Return water temp is the primary determinant of combustion efficiency. The firing rate is secondary, but still matters, but when the return water temps are well below the setpoint, the cooler return water temp more than offsets the efficiency hit from the higher firing rate.

It's not OK to have running a dozen 1-minute burns per hour. Sounds like they don't have the same hysteresis designed in when controlling it by return water temp. If you can find a return water temp where it gives you 3+ minute burns and doesn't sound like fighter aircraft on a takeoff roll when cold starting a zone, that would probably be a reasonable operating point.

If you can set it up with a fixed temp 125-130F output and ignore the outdoor reset it will still deliver condensing efficiency and probably won't short-cycle itself to death, depending on how much built in temperature hysteresis it allows.

As for alternatives, since it seems you can heat the place even with 130F water, if you have the space for it, a 48 gallon AO Smith Vertex (or a 34 gallon Polaris) HW heater and an isolating plate-type heat exchanger would likely be able to handle your loads and still run in the low-mid 90s for efficiency. This takes a bit of design work to get right, but it's not a rocket science type hydronic design problem. The Polaris controls have a very tight hysteresis, but that can be modified with a fairly easy resistor hack if you are comfortable with a soldering iron and tinkering with electronics. I'm not sure if that's the case with the Vertex. Both water heaters have side ports on the tank specifically designed for combi heating systems. The thermal mass of the water in the tank is an order of magnitude higher than in your fin-tube baseboard system, and even without a 20F+ hysteresis it can get there without short-cycling.

If you can set it up with a fixed temp 125-130F output and ignore the outdoor reset it will still deliver condensing efficiency and probably won't short-cycle itself to death, depending on how much built in temperature hysteresis it allows.

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Interesting that you mention this. I have actually had it set up at 130F fixed supply temp for a little while (since 12/23). It seems to work well. Even with the weather being fairly warm, the burn times are still good and the burner isn't shutting down on high limit during calls for heat. It comes up to 130F fairly quick, then modulates to minimum fire, and the temp slowly creeps up to about 135-138F during a 10-12 minute burn (that's with outside temps in the upper 30s / low 40s). I'll get to see how it does in the cold - it's supposed to be in the teens later this week with one night about -5F.

Now, I just got my brand new replacement boiler installed today, and I'm more frustrated than ever. The new boiler acts exactly like the old boiler - meaning I get a weird buzzing/vibration from the main gas valve at low fire (which is quite a bit of the time), and on a call for heat the burner will sometimes give 5-8 short 1-second 'bursts' of flame at start-up before it finally ignites and stays burning. Both Navien and the installer are stumped. They say they haven't seen this before.

However, my propane supplier took one look at the piping and said the gas supply piping is undersized. This is based on the max input of 150,000 BTU/hr. What's weird is that the unit seems to have no problem firing at max rate to make DHW. And my gas valve buzzing issue only happens when the burner modulates way down when heating. I would think that if I had a gas supply issue I'd see problems at max fire & ignition when making DHW.

Anyhow, my money is on how the gas is entering the main gas valve - seems like there is something unusual about the flow pattern entering the main gas valve.